AIR-GERMS AND SPONTANEOUS GENERATION. 99 



could be kept for an indefinite time in the stove, without giving rise 

 to the smallest production of mould or infusoria. The milk preserves 

 its taste, its smell, and all its properties ; and the atmosphere of the 

 flask is only slightly modified in its composition. This difference be- 



tween milk and urine, or sweetened yeast-water, must be attributed 

 to the alkaline condition of the former medium, whereas the two oth- 

 ers are acid. In fact, if we previously neutralize the acid of the sweet- 

 ened yeast-water, by means of calcium carbonate, we obtain organisms 

 under the same conditions of the experiment as those under which 

 they were not before developed. 



